With the sophistication of optical communication systems, demand has been increased for highly functional optical modules (optical components). A planar lightwave circuit can be implemented in the form of various lightwave circuits by forming optical waveguides on the substrate thereof, and is being used as a component composing an optical module. For the higher functionalization of optical modules, hybrid optical modules are being implemented by integrating planar lightwave circuits having different functions or by integrating a spatial optical component, such as a lens and a spatial phase modulator, and a planar lightwave circuit. Specific examples of optical modules include, for instance, an RZ-DQPSK (Return to Zero Differential Quadrature Phase Shift Keying) module obtained by optically coupling planar lightwave circuits made of different materials such as silica-based glass and lithium niobate (LN).
As an example, an optical module will be assumed that a first planar lightwave circuit made of silica-based glass and a second planar lightwave circuit made of LN are butt jointed. FIG. 1 shows a conventional example of PTL 1. Chances are that reflection is caused on an interface 3 between a first planar lightwave circuit 1 and a second planar lightwave circuit 2. To prevent this, (1) first, an angle θ is determined such that Fresnel reflection R is not coupled, as a return light, to an optical waveguide 4 of the first planar lightwave circuit 1. In general, the angle θ is an angle of 4 to 12 degrees. Here, the Fresnel equation is expressed by the following equation. n1 and na are respectively refractive indices of silica-based glass and LN. It should be noted that, where an adhesive is applied between the first planar lightwave circuit and the second planar lightwave circuit, na is set as the refractive index of the adhesive.
                    R        =                              (                                                            n                  1                                -                                  n                  a                                                                              n                  1                                +                                  n                  a                                                      )                    2                                    [                  Math          .                                          ⁢          1                ]            
(2) Next, an angle φ of an optical waveguide 5 of the second planar lightwave circuit 2 is determined such that the Snell's law expressed by the following equation is satisfied. n2 is the refractive index of LN.
                                          sin            ⁢                                                  ⁢            θ                                sin            ⁢                                                  ⁢            φ                          =                              n            2                                n            1                                              [                  Math          .                                          ⁢          2                ]            
In FIG. 1, the joint interface 3 is slanted by an angle α1 with respect to a sidewall 6 of the planar lightwave circuit. However, when the optical waveguide is slanted in the vicinity of the interface, the angle formed by the joint interface 3 and the sidewall 6 of the planar lightwave circuit can be set to the right angle while reflection can be prevented. When planar lightwave circuits are butt jointed on the end surfaces thereof perpendicular to the lateral surfaces thereof, the planar lightwave circuits can be formed in rectangular shapes with four right-angled corners and can be easily processed. FIG. 2 shows such an example. An optical fiber 3, the first planar lightwave circuit 1 and the second planar lightwave circuit 2 are butt jointed, while being aligned on an alignment member 4. In this example, the first planar lightwave circuit 1 and the second planar lightwave circuit 2 are respectively formed in oblong shapes. Although reference signs are different from those in FIG. 1, the angle formed by the optical waveguide of the first planar lightwave circuit 1 and a normal of the interface is set as φ1, whereas the angle formed by the optical waveguide of the second planar lightwave circuit 2 and a normal of the interface is set as θ1. In this case, the Snell's law is expressed as the following equation.
                                          sin            ⁢                                                  ⁢                          ϕ              1                                            sin            ⁢                                                  ⁢                          θ              1                                      =                              n            2                                n            1                                              [                  Math          .                                          ⁢          3                ]            
Based on the aforementioned conditions, φ1 and θ1 are generally set to 6 degrees and 4 degrees, respectively in the butt joint between the first planar lightwave circuit made of silica-based glass and the second planar lightwave circuit made of LN.